There are now some 30,000 Alpha courses running around the world, say organisers. Sessions are held in prisons, workplaces, schools, colleges and military establishments.

Around 7,000 UK churches are signed up, many running several courses a year.

The number of converts does not match the tide of Christians leaving the church, says Alpha communications director Mark Elsdon-Dew.

Critics

But Alpha predominantly attracts young people - aged 27 on average - a good sign for the future, he suggests.

A 'supper' held at Holy Trinity Brompton (HTB) church - the home of Alpha - marks the end of the latest 10-week course.

I am welcomed with beaming smiles and enthusiastic handshakes, and sat at a table of 10 tucking into salmon, couscous and wine.

Abigail said her Christian family were delighted she found her faith

Alpha has been praised by church leaders and boasts celebrity endorsements.

But it has also attracted criticism as a "Coca Cola" version of Christianity, over the way it is marketed.

Some say Alpha is more a rich social club than a Christian coming-together. Others are uncomfortable with what they call its prescriptive teaching and attitudes towards homosexuality and other faiths.

'What made you come here?' I ask 'graduate' Abigail, 30, who tells me she was the "rebel" in a Christian family until recently.

It's a complex tale, involving a break-up with a boyfriend and the disappearance of a benign cyst in her eye following a prayer to God.

Alpha male

Soon after she moved in with her sister, who is married to a curate at HTB, got a job there and became involved.

"Everything has fallen into place. Before I said I was happy, doing my own thing. I thought Christianity was fine for my family but not me.

"But after I said the prayer in the second week of the course that was it."

Another woman says she was already a Christian but felt there was something missing.

God sent me a lifeline - there was a guy in the gym, a violent drugs baron, who had turned his life around through faith

Simon Pinchbeck, ex police officer

She breaks down while describing the Holy Spirit weekend - a part of the course which involves, in the words of one course-goer "experiencing the love of God directly".

"I feel like a whole person for the first time. And all because I found God."

Not all are convinced. One man said: "It was interesting but I don't feel I have been successful in building a relationship with God."

There's a collective groan as he admits he did not attend the away weekend, which many see as a crucial.

But he is applauded when he adds: "I'll do it in November and we'll see what happens."

Nicky Gumbel, Alpha's creator in its present form, spreads the message in person or via videos around the world. The Alpha male, so to speak.

He says Alpha is growing because there's a "spiritual hunger in every heart".

"It's hard to find a place in the modern world where you can discuss it in an unthreatening environment, where there are people just like you. People tell friends and it grows.

Clubs and drugs

"You see how it changes people's lives. We want the divorce rate down, crime down, the prison population reduced. Politicians can only change certain things. They can't change people's hearts."

Talking of crime, ex-Metropolitan Police officer Simon Pinchbeck, 47, is less typical of the wealthy set HTB has a reputation for attracting.

It was like to talking to a politician of the opposite persuasion - they were all very nice but I came out feeling exactly as I did when I went in

Jill Ross, teacher

At 40, he says, his life spiralled out of control. He started clubbing, taking drugs and left his family.

"I got pensioned off from the police. I was bitter. I got into the wrong crowd and turned to making a living out of crime.

"Then God sent me a lifeline. There was a guy in the gym, a violent drugs baron, who had turned his life around through faith. He looked so peaceful. I said I've got to have some of what he's got.

"Coming here was like nothing I've ever experienced. The feeling when the spirit of God entered my life was unbelievable."

Alpha staff are the first to admit its style is not for everyone - a view Jill Ross, 47, a teacher from Hertfordshire, agrees with.

'Mind games'

She happily admits she is an agnostic who began going to church while trying to fulfil the criteria to get her daughter into the local primary school.

She was eventually persuaded by friends to try Alpha, but said she felt it did not answer her questions about Christianity.

"It was like to talking to a politician of the opposite persuasion. They were all very nice people, but I came out feeling exactly as I did when I went in," said Jill, who asked that her name be changed.

Among her problems with Alpha was her feeling that other faiths were not given equal status.

"I felt it was biased against other religions, whereas I believe all roads lead to one God," she said.

There was also an element of "therapy" about the style of the course, she said.

"It plays huge mind games with people. You have to be a strong personality to resist, but I did. They were working on me all the time. They said they would pray for me.

"I think they tend to prey on who is vulnerable. The whole thing for people is about being accepted and feeling like they belong.